The present invention relates to electrical isolation devices. In particular, the present invention is a device for providing an electrically isolated output current whose magnitude is a function of the duty cycle of a time varying input signal.
Electrical isolation of signals or circuits is required in a wide variety of applications. Electrical isolation may be required due to differences in operating voltages between the source of the input signal and the signal processing circuitry, for noise consideration, or for a wide variety of other considerations.
One widely used technique of isolating circuits or portions of circuits is by means of an optical isolator. A typical optical isolator includes a light source, such as light emitting diode, and a light sensor, such as a phototransistor. The light emitting diode and phototransistor are often packaged in the same integrated circuit package, but are electrically isolated from one another. The light emitting diode is driven by an input signal to produce light, and the phototransistor provides an output which is a function of the light received from the light emitting diode.
It is important that isolation circuitry not introduce errors. This can present a problem with optical couplers, since the gain characteristics of a phototransistor can vary from unit to unit. In addition, the characteristics of the phototransistor often vary as a function of temperature, thereby making the accuracy of the optical isolator circuitry temperature dependent.